It is known that the action to be exerted on the pack of the clutch diks is made easier by means of a hydraulic circuit which comprises a pump and a piston acting on the disks' pack. In the motorcycles, for example, a lever is provided for operating said pump whose activation determines the displacement of the piston.
In the motorcycles provided with a traditional apparatus of this type, a drawback is due to the relative difficulty of adjusting the idle stroke of the control lever. In fact, in the existing devices, the lever is connected to a piston sliding into a respective chamber exhibited by the pump and, in order to adjust the idle stroke of the lever it is currently necessary to move axially the piston with respect to the chamber inside which it slides. This approach is however very complex structurally, and affects negatively the constructional simplicity of the device and its robustness upon use, besides raising the relevant production cost thereof.